Many peripheral working aids are already available on the computer market to help the user control the cursor on the screen. The most popular of these is the so-called "mouse" whose movement controls the movement of the cursor. Most of the mice hitherto available operated by optical or mechanical processes and the positions were determined by means of a special indicator board with special support for the mice or with wires stretched across the indicator board. Since this requires relatively elaborate mechanical work or the formation of special surfaces, which increase enormously in cost with the increasing requirements for accuracy and power of resolution, there has for some time now been a quest for different systems for determining position.
The present invention relates to an apparatus for determining the position of a movable object within a given boundary and relatively to fixed reference points by means of acoustic signals, preferably ultrasonic pulses, comprising transmitter and receptor means for the ultrasonic pulses and means for determining their travelling time between the movable object and the fixed reference points.
An apparatus of this kind is described in the article "Zweiohrige Maus" (two-eared mouse) by P. Wendt in "mc Die Mikrocomputer Zeitschrift", No. 10/1984. In this apparatus, the receiver means are microphones and to measure the travelling time, counters are started in response to a trigger signal when sound pulses are transmitted by the transmitter and stopped on reception of a signal by the microphones. Care must be taken to ensure that the outlet opening of the transmitter is directed towards the microphones because otherwise the travelling time may be determined by reflections. The position determination is not very accurate, partly for this reason and partly owing to the relatively large size of the microphones. This known apparatus therefore cannot be used as so-called digitizer, i.e., as peripheral instruments for feeding the coordinates of a drawing into a computer system for geometrical data processing.
It is an object of the present invention to provide an apparatus for position determination distinguished by very precise determination of the position and a high power of resolution.